1. Field of the Invention
The present invention relates to wireless communications, and more specifically, it relates to radio-frequency (RF) and free-space optical (FSO) communications.
2. Description of Related Art
The availability of free-space optical links is limited by atmospherics and weather conditions. RF and microwave systems are less susceptible to these problems. Hybrid RF/Optical communications systems that can switch back and forth between RF and Optical transmissions in order to optimize the overall availability of the link and maximize communication performance would be a solution. The use of a terminal with shared RF/Optical aperture could provide the overall link availability advantage described above in the smallest form factor possible. This becomes increasingly important in deployment scenarios where space is at a premium and the use of multiple RF and optical apertures is not desirable, for example on small aircraft, satellites, and in certain ground vehicles and ground deployments.
As discussed in U.S. Pat. No. 6,667,831, incorporated herein by reference, FIG. 1 illustrates a traditional Gregorian telescope 100 according to the prior art. The Gregorian telescope 100 has a concave primary mirror 102 and a concave secondary mirror 104. In many traditional Gregorian telescopes, the primary mirror has a parabolic curvature and the secondary mirror has an elliptical curvature. The secondary mirror 104 is disposed outside the focal plane of the primary mirror 102, and the mirrors share a common optical axis 106. The primary mirror 102 reflects light from a far field and directs the light towards the secondary mirror 104. The secondary mirror 104 is appropriately sized and positioned so that light reflecting off the primary mirror 102 is incident on the secondary mirror 104. The secondary mirror 104 reflects light and directs it through an aperture 108 in the primary mirror 102 that is centered about the optical axis 106. The light is thereafter imaged at the focal plane 110 of the compact telescope for advantageous use.
FIG. 2 illustrates an embodiment of a compact telescope as discussed in U.S. Pat. No. 6,667,831. The compact telescope 200 comprises a first reflecting surface 202 and a radially defined second reflecting surface 204. The first reflecting surface 202 includes an annular outer portion 206, a radially defined inner portion 208, and a radially defined aperture 210. Other shapes may be used for these elements of the compact telescope, however, alternative shapes may increase the complexity of the optics.
The outer portion 206 of the first reflecting surface 202 is the functional equivalent of the primary mirror in a traditional Gregorian telescope, while the inner portion 208 is the functional equivalent of the secondary mirror. Therefore, hereinafter, the term “primary mirror”, as it relates to a compact telescope, is used interchangeably with the outer portion 206 of the first reflecting surface. Likewise, the term “secondary mirror”, as it relates to a compact telescope, is used interchangeably with the inner portion 208 of the first reflecting surface. The primary and secondary mirrors 206, 208 are both concave, with the curvature of the secondary mirror 208 being greater than the curvature of the primary mirror 206. In FIG. 2, both the primary mirror 206 and the secondary mirror 208 have elliptical curvatures (i.e., conic between −1 and 0). Those skilled in the art will recognize that with both mirrors having elliptical curvatures, correcting for both spherical and coma aberrations is facilitated without the need for additional optical elements. In an alternative embodiment, the primary mirror 206 may have a parabolic curvature (i.e., conic equal to −1) and the secondary mirror 208 may have an elliptical curvature. Other curvatures may also be used for the primary and secondary mirrors 206, 208 of the compact telescope.
The optical axes 212 of the primary and secondary mirrors 206, 208 are coincidental. Additionally, the aperture 210 and the second reflecting surface 204 are centered upon the coincident optical axes 212. Non-coincidental and/or off-axis optics may he employed, however, coincident optical axes reduce complications in aligning the optical elements and simplify the optics of the compact telescope.
In the embodiment of FIG. 2, the primary and secondary mirrors 206, 208 form the integral first reflecting surface 202. Such a double-curved mirror facilitates manufacturing and optical axis alignment of each curvature on the first reflecting surface 202. This is important because greater errors in axis alignment result in greater optical aberrations. For example, a double-curved mirror may be manufactured using diamond turning or other appropriate equipment that is frequently used to create high quality mirrors. With the appropriate manufacturing equipment, the primary and secondary mirrors may he manufactured sequentially using a single piece of equipment without realigning the equipment to obtain coincidental optical axes.
Alternatively, in lieu of a double curved mirror, the compact telescope may comprise a first reflecting surface having an annular shape (the primary mirror), with a third reflecting surface (the secondary mirror) disposed within the inner radius of the first reflecting surface. The curvatures of this alternative embodiment for the first and third reflecting surfaces are the same as the curvatures for the aforementioned outer and inner portions, respectively.
Returning to FIG. 2, the second reflecting surface 204 is a planar surface, hereinafter referred to as the “folding mirror”. The folding mirror 204 optically couples the primary mirror 206 to the secondary mirror 208. The folding mirror 204 is disposed between the first reflecting surface 202 and the focal plane of the primary mirror 206. Thus, light from a far field may enter the primary aperture of the compact telescope 200 and reflect off the primary mirror 206 towards the folding mirror 204. The folding mirror 204 reflects such light towards the secondary mirror 208, and the secondary mirror 208 reflects the light back towards the folding mirror 204. Upon this second reflection from the folding mirror 204, the light passes through the aperture 210. Light emerging from the aperture 210 creates an upright image at the focal plane 214 of the compact telescope that may be advantageously used.
Alternative embodiments of the compact telescope may include a curved folding mirror. A curved folding mirror preferably has a high radius of curvature, such as a radius of 1 meter or more. Smaller curvatures may also be employed. In another alternative embodiment, the folding mirror comprises a steering mirror. The steering mirror may have a planar or curved reflective surface. A steering mirror having a curved reflective surface may help improve the optics of a compact telescope when the optical axes of the primary and secondary mirrors are imprecisely aligned.